Method and apparatus for determining number of harq processes in wireless communication system

ABSTRACT

Provided are a method for determining the number of hybrid automatic repeat request (HARQ) processes in a carrier aggregated system configured with a plurality of serving cells, and an apparatus using such a method. The method receives data from a downlink subframe of a second serving cell, and transmits an ACK/NACK signal for the data from an uplink subframe of a first serving cell, wherein the first serving cell uses a first-type frame, the second serving cell uses a second-type frame, and the number of HARQ processes in the second serving cell are determined with respect to each subframe comprised in the second-type frame and on the basis of the number of downlink subframes comprised in each section comprising a set number of subframes.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and more particularly, to a method and apparatus for determining the number of hybrid automatic repeat request (HARQ) processes in a wireless communication system in which serving cells using a radio frame of different types are aggregated.

Related Art

Long term evolution (LTE) based on 3rd Generation Partnership Project (3GPP) Technical Specification (TS) is one of mobile communication standards. Meanwhile, 3GPP LTA-advanced (LTE-A) which is an evolution of 3GPP LTE is currently underway. A technique introduced in 3GPP LTE-A is a carrier aggregation.

A carrier aggregation uses a plurality of component carriers. A component carrier is defined by the center frequency and a bandwidth. One downlink component carrier or a pair of an uplink component carrier and a downlink component carrier correspond to one cell. It can be said that a terminal being served using a plurality of downlink component carriers is being served from a plurality of serving cells.

Meanwhile, an error compensation technique, which is to secure the reliability of wireless communications, includes a forward error correction (FEC) scheme and automatic repeat request (ARQ) scheme. In the forward error correction (FEC) scheme, an error at a reception unit is corrected by adding extra error correction codes to information bits. The FEC scheme has an advantage in that there is less time delay and no information required to be transmitted and received between a transmission unit and a reception unit, however, it is a weak point that system efficiency is low in good channel conditions. The ARQ scheme is strong at transmission reliability, however, it causes time delay and the system efficiency is low in poor channel conditions.

The hybrid automatic repeat request (HARQ) is a scheme in which FEC and ARQ are coupled, it checks whether to include an error that data received by a physical layer cannot be decoded, and if the error is occurred, efficiency can be increased by requesting retransmission.

A receiver in HARQ, if an error is not detected in data received, informs a success of the reception by transmitting an acknowledgement signal to the reception acknowledgement. If an error is detected in data received, a receiver informs a transmitter of the error detected by transmitting a NACK signal to the reception acknowledgement. If the NACK signal is received, the transmitter may retransmit data.

Meanwhile, in the carrier aggregation in of the next generation wireless communication systems, serving cells using TDD and serving cells using FDD may be aggregated. That is, a plurality of serving cells that uses different types of radio frames may be allocated to a UE. In a conventional art, it is defined to aggregate the serving cells that use identical type of radio frames only in carrier aggregation. Accordingly, it is not considered of a method that determines the number of HARQ processes in aggregation of serving cells that use different types of radio frames.

In case that serving cells that use different types of radio frames are aggregated, it is problematic in which way the number of HARQ processes, which are simultaneously capable, is determined.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for determining the number of HARQ processes in wireless communication systems in which a plurality of serving cells that uses different types of radio frames is aggregated.

In one aspect, provided is a method for determining a number of hybrid automatic repeat request (HARQ) processes in a carrier aggregation system in which a plurality of serving cells are configured. The method includes receiving data in a downlink subframe of a second serving cell and transmitting an ACK/NACK signal in response to the data in an uplink subframe of a first serving cell. A first type of frame is used in the first serving cell, and a second type of frame is used in the second serving cell. The number of HARQ processes of the secondary cell is determined based on the number of downlink subframes that are included in each of sections including a specific number of subframes based on each subframe included in the second type of frame.

In another aspect, an apparatus is provided. The apparatus includes a radio frequency (RF) unit configured to transmit and receive a radio signal and a processor connected to the RF unit. The processor is configured for receiving data in a downlink subframe of a second serving cell and transmitting an ACK/NACK signal in response to the data in an uplink subframe of a first serving cell. A first type of frame is used in the first serving cell, and a second type of frame is used in the second serving cell. The number of HARQ processes of the secondary cell is determined based on the number of downlink subframes that are included in each of sections including a specific number of subframes based on each subframe included in the second type of frame.

In wireless communication systems in which a plurality of serving cells that uses different types of radio frames is aggregated, the number of HARQ processes of secondary cells can be effectively determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of an FDD radio frame.

FIG. 2 shows the structure of a TDD radio frame.

FIG. 3 shows an example of a resource grid for one downlink slot.

FIG. 4 shows the structure of a DL subframe.

FIG. 5 shows the structure of an UL subframe.

FIG. 6 shows the channel structure of a PUCCH format 1b in a normal CP.

FIG. 7 shows the channel structure of PUCCH formats 2/2a/2b in a normal CP.

FIG. 8 illustrates the channel structure of a PUCCH format 3.

FIG. 9 shows an example of a comparison between a single carrier system and a carrier aggregation system.

FIG. 10 shows one example in which a plurality of serving cells uses different types of radio frames in a wireless communication system.

FIG. 11 shows a method of transmitting ACK/NACK for downlink data received through a primary cell.

FIG. 12 shows a method of transmitting ACK/NACK for downlink data received through a secondary cell.

FIG. 13 shows an example of ACK/NACK transmission timing when a primary cell is an FDD cell and a secondary cell is a TDD cell.

FIG. 14 shows an ACK/NACK transmission method based on Method 1.

FIG. 15 shows another example of ACK/NACK transmission timing when a primary cell is an FDD cell and a secondary cell is a TDD cell.

FIG. 16 shows an ACK/NACK transmission method based on Method 2.

FIG. 17 illustrates a DL HARQ that is performed in a cell of 3GPP LTE.

FIG. 18 illustrates an example of determining the number of DL HARQ processes in a FDD cell.

FIG. 19 illustrates an example of determining the number of DL HARQ processes for each of UL-DL configurations 0 to 2 in a TDD cell.

FIG. 20 illustrates an example of determining the number of DL HARQ processes for each of UL-DL configurations 3 to 5 in a TDD cell.

FIG. 21 illustrates an example of determining the number of DL HARQ processes for UL-DL configuration 6 a TDD cell.

FIG. 22 illustrates an example of determining the number of DL HARQ processes of the secondary cell.

FIG. 23 is a block diagram of a wireless apparatus in which the embodiments of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

User Equipment (UE) can be fixed or can have mobility. UE can also be called another term, such as a Mobile Station (MS), a Mobile Terminal (MT), a User Terminal (UT), a Subscriber Station (SS), a wireless device, a Personal Digital Assistant (PDA), a wireless modem, or a handheld device.

The BS commonly refers to a fixed station that communicates with UE. The BS can also be called another term, such as an evolved-NodeB (eNodeB), a Base Transceiver System (BTS), or an access point.

Communication from a BS to UE is called downlink (DL), and communication from UE to a BS is called uplink (UL). A wireless communication system including a BS and UE can be a Time Division Duplex (TDD) system or a Frequency Division Duplex (FDD) system. A TDD system is a wireless communication system that performs UL and DL transmission/reception using different times in the same frequency band. An FDD system is a wireless communication system that enables UL and DL transmission/reception at the same time using different frequency bands. A wireless communication system can perform communication using radio frames.

FIG. 1 shows the structure of an FDD radio frame.

The FDD radio frame includes 10 subframes, and one subframe includes two consecutive slots. The slots within the radio frame are assigned indices 0-19. The time that is taken for one subframe to be transmitted is called a Transmission Time Interval (TTI). A TTI can be a minimum scheduling unit. For example, the length of one subframe can be 1 ms, and the length of one slot can be 0.5 ms. Hereinafter, the FDD radio frame may be simply referred to as an FDD frame.

FIG. 2 shows the structure of a TDD radio frame.

Referring to FIG. 2, a downlink (DL) subframe and an uplink (UL) subframe coexist in a TDD radio frame used in TDD. Table 1 shows an example of a UL-DL configuration of the radio frame.

TABLE 1 Uplink- Downlink- downlink to-uplink config- switch-point Subframe n uration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U U 2 5 ms D S U D D D S U D D 3 10 ms  D S U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D D D 6 5 ms D S U U U D S U U D

In Table 1, ‘D’ indicates a DL subframe, ‘U’ indicates a UL subframe, and ‘S’ indicates a special subframe. When a UL-DL configuration is received from a BS, a UE can be aware of whether each subframe in a radio frame is a DL subframe or a UL subframe. Hereinafter, reference can be made to Table 1 for a UL-DL configuration N (N is any one of 0 to 6).

In the TDD frame, a subframe having an index #1 and an index #6 may be a special subframe, and includes a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS). The DwPTS is used in initial cell search, synchronization, or channel estimation in UE. The UpPTS is used for channel estimation in a BS and for the uplink transmission synchronization of UE. The GP is an interval in which interference occurring in UL due to the multi-path delay of a DL signal between UL and DL is removed. Hereinafter, the TDD radio frame may be simply referred to as a TDD frame.

FIG. 3 shows an example of a resource grid for one downlink slot.

Referring to FIG. 3, the downlink slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbol in the time domain and includes N₁ Resource Blocks (RBs) in the frequency domain. The RBs includes one slot in the time domain and a plurality of consecutive subcarrier in the frequency domain in a resource allocation unit. The number of RBs N_(RB) included in the downlink slot depends on a downlink transmission bandwidth N^(DL) configured in a cell. For example, in an LTE system, the N₁ can be any one of 6 to 110. An uplink slot can have the same structure as the downlink slot.

Each element on the resource grid is called a Resource Element (RE). The RE on the resource grid can be identified by an index pair (k,l) within a slot. Here, k (k=0, N_(RB)×12−1) is a subcarrier index within the frequency domain, and 1 (1=0, . . . , 6) is an OFDM symbol index within the time domain.

Although 7×12 REs including 7 OFDM symbols in the time domain and 12 subcarrier in the frequency domain have been illustrated as being included in one RB in FIG. 3, the number of OFDM symbols and the number of subcarriers within an RB are not limited thereto. The number of OFDM symbols and the number of subcarriers can be changed in various ways depending on the length of a CP, frequency spacing, etc. In one OFDM symbol, one of 128, 256, 512, 1024, 1536, and 2048 can be selected and used as the number of subcarriers.

FIG. 4 shows the structure of a DL subframe.

Referring to FIG. 4, a downlink (DL) subframe is divided into a control region and a data region in the time domain. The control region includes a maximum of former 3 (maximun 4 according to circumstances) OFDM symbols of a first slot within a subframe, but the number of OFDM symbols included in the control region can be changed. A control channel different from a physical downlink control channel (PDCCH) is allocated to the control region, and a physical downlink shared channel (PDSCH) is allocated to the data region.

As disclosed in 3GPP TS 36.211 V8.7.0, in 3GPP LTE, physical channels can be divided into a physical downlink shared channel (PDSCH) and a physical uplink shared channel (PUSCH), that is, data channels, and a physical downlink control channel (PDCCH), a physical control format indicator channel (PCFICH), a physical hybrid-ARQ indicator channel (PHICH), and a physical uplink control channel (PUCCH), that is, control channels.

A PCFICH that is transmitted in the first OFDM symbol of a subframe carries a Control Format Indicator (CFI) regarding the number of OFDM symbols (i.e., the size of a control region) that are used to send control channels within the subframe. UE first receives a CFI on a PCFICH and then monitors PDCCHs. Unlike in a PDCCH, a PCFICH is not subject to blind decoding, but is transmitted through the fixed PCFICH resources of a subframe.

A PHICH carries a positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal for an uplink Hybrid Automatic Repeat reQuest (HARD). An ACK/NACK signal for uplink (UL) data on a PUSCH which is transmitted by UE is transmitted on a PHICH.

A physical broadcast channel (PBCH) is transmitted in the former 4 OFDM symbols of a second slot within the first subframe of a radio frame. The PBCH carries system information that is essential for UE to communicate with a BS, and system information transmitted through a PBCH is called a Master Information Block (MIB). In contrast, system information transmitted on a PDSCH indicated by a PDCCH is called a System Information Block (SIB).

Control information transmitted through a PDCCH is called Downlink Control Information (DCI). DCI can include the resource allocation of a PDSCH (this is also called a DL grant), the resource allocation of a PUSCH (this is also called an UL grant), a set of transmit power control commands for individual MSs within a specific UE group and/or the activation of a Voice over Internet Protocol (VoIP).

FIG. 5 shows the structure of an UL subframe.

Referring to FIG. 5, the UL subframe can be divided into a control region to which a physical uplink control channel (PUSCH) for carrying uplink control information is allocated and a data region to which a physical uplink shared channel (PUSCH) for carrying user data is allocated in the frequency domain.

A PUCCH is allocated with an RB pair in a subframe. RBs that belong to an RB pair occupy different subcarriers in a first slot and a second slot. An RB pair has the same RB index m.

In accordance with 3GPP TS 36.211 V8.7.0, a PUCCH supports multiple formats. A PUCCH having a different number of bits in each subframe can be used according to a modulation scheme that is dependent on a PUCCH format.

Table 2 below shows an example of modulation schemes and the number of bits per subframe according to PUCCH formats.

TABLE 2 PUCCH format Modulation scheme Number of bits per subframe 1 N/A N/A 1a BPSK 1 1b QPSK 2 2 QPSK 20 2a QPSK + BPSK 21 2b QPSK + QPSK 22

The PUCCH format 1 is used to send a Scheduling Request (SR), the PUCCH formats 1a/1b are used to send an ACK/NACK signal for an HARQ, the PUCCH format 2 is used to send a CQI, and the PUCCH formats 2a/2b are used to send a CQI and an ACK/NACK signal at the same time. When only an ACK/NACK signal is transmitted in a subframe, the PUCCH formats 1a/1b are used. When only an SR is transmitted, the PUCCH format 1 is used. When an SR and an ACK/NACK signal are transmitted at the same time, the PUCCH format 1 is used. In this case, the ACK/NACK signal is modulated into resources allocated to the SR and is then transmitted.

All the PUCCH formats use the Cyclic Shift (CS) of a sequence in each OFDM symbol. A CS sequence is generated by cyclically shifting a base sequence by a specific CS amount. The specific CS amount is indicated by a CS index.

An example in which a base sequence r_(u)(n) has been defined is the same as the following equation.

r _(u)(n)=e ^(jb(n)π/4)  [Equation 1]

Here, u is a root index, n is an element index wherein 0≤n≤N−1, and N is the length of the base sequence. b(n) is defined in section 5.5 of 3GPP TS 36.211 V8.7.0.

The length of a sequence is the same as the number of elements included in the sequence. U can be determined by a cell identifier (ID), a slot number within a radio frame, etc.

Assuming that a base sequence is mapped to one resource block in the frequency domain, the length N of the base sequence becomes 12 because one resource block includes 12 subcarriers. A different base sequence is defined depending on a different root index.

A CS sequence r(n, I_(cs)) can be generated by cyclically shifting the base sequence r(n) as in Equation 2.

$\begin{matrix} {{{r\left( {n,I_{cs}} \right)} = {{r(n)} \cdot {\exp \left( \frac{j\; 2\pi \; I_{cs}n}{N} \right)}}},{0 \leq I_{cs} \leq {N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack \end{matrix}$

Here, I_(cs) is a CS index indicative of a CS amount (0≤I_(cs)≤N−1).

An available CS index of a base sequence refers to a CS index that can be derived from the base sequence according to a CS interval. For example, the length of a base sequence is 12 and a CS interval is 1, a total number of available CS indices of the base sequence becomes 12. Or, if the length of a base sequence is 12 and a CS interval is 2, a total number of available CS indices of the base sequence becomes 6.

FIG. 6 shows the channel structure of the PUCCH format 1b in a normal CP.

One slot includes 7 OFDM symbols, the 3 OFDM symbols become Reference Signal (RS) OFDM symbols for a reference signal, and the 4 OFDM symbols become data OFDM symbols for an ACK/NACK signal.

In the PUCCH format 1b, a modulation symbol d(0) is generated by performing Quadrature Phase Shift Keying (QPSK) modulation on an encoded 2-bit ACK/NACK signal.

A CS index I_(cs) can vary depending on a slot number ‘ns’ within a radio frame and/or a symbol index ‘1’ within a slot.

In a normal CP, 4 data OFDM symbols for sending an ACK/NACK signal are present in one slot. It is assumed that corresponding CS indices in respective data OFDM symbols are I_(cs0), I_(cs1), I_(cs2), and I_(cs3).

The modulation symbol d(0) is spread into a CS sequence r(n,Ics). Assuming that a 1-dimensional spread sequence corresponding to an (i+1)^(th) OFDM symbol is m(i) in a slot,

{m(0), m(1), m(2), m(3)}={d(0)r(n,I_(cs0)), d(0)r(n,I_(cs1)), d(0)r(n,I_(cs2)), d(0)r(n,I_(cs3))} can be obtained.

In order to increase a UE capacity, the 1-dimensional spread sequence can be spread using an orthogonal sequence. The following sequence is used as an orthogonal sequence w_(i)(k) (i is a sequence index, 0≤k≤K−1) wherein a spreading factor K=4.

TABLE 3 Index (i) [w_(i)(0), w_(i)(1), w_(i)(2), w_(i)(3)] 0 [+1, +1, +1, +1] 1 [+1, −1, +1, −1] 2 [+1, −1, −1, +1]

The following sequence is used as an orthogonal sequence w_(i)(k) (i is a sequence index, 0≤k≤K−1) wherein a spreading factor K=3.

TABLE 4 Index (i) [w_(i)(0), w_(i)(1), w_(i)(2)] 0 [+1, +1, +1] 1 [+1, e^(j2π/3), e^(j4π/3)] 2 [+1, e^(j4π/3), e^(j2π/3)]

A different spreading factor can be used in each slot.

Accordingly, assuming that a specific orthogonal sequence index i is given, 2-dimensional spread sequences {s(0), s(1), s(2), s(3)} can be expressed as follows.

{s(0), s(1), s(2), s(3)}={w_(i)(0)m(0), w_(i)(1)m(1), w_(i)(2)m(2), w_(i)(3)m(3)}

The 2-dimensional spread sequences {s(0), s(1), s(2), s(3)} are subject to IFFT and then transmitted in a corresponding OFDM symbol. Accordingly, an ACK/NACK signal is transmitted on a PUCCH.

A reference signal having the PUCCH format 1b is also transmitted by spreading the reference signal into an orthogonal sequence after cyclically shifting a base sequence r(n). Assuming that CS indices corresponding to 3 RS OFDM symbols are I_(cs4), I_(cs5), and I_(cs6), 3 CS sequences r(n,I_(cs4)), r(n,I_(cs5)), r(n,I_(cs6)) can be obtained. The 3 CS sequences are spread into an orthogonal sequence w^(RS) _(i)(k) wherein K=3.

An orthogonal sequence index i, a CS index I_(cs), and an RB index m are parameters necessary to configure a PUCCH and are also resources used to classify PUCCHs (or MSs). If the number of available CSs is 12 and the number of available orthogonal sequence indices is 3, a PUCCH for a total of 36 MSs can be multiplexed with one RB.

In 3GPP LTE, a resource index n⁽¹⁾ _(PUCCH) is defined so that UE can obtain the three parameters for configuring a PUCCH. The resource index n⁽¹⁾ _(PUCCH)=n_(CCE)+N⁽¹⁾ _(PUCCH), wherein n_(CCE) is the number of the first CCE used to send a corresponding PDCCH (i.e., PDCCH including the allocation of DL resources used to received downlink data corresponding to an ACK/NACK signal), and N⁽¹⁾ _(PUCCH) is a parameter that is informed of UE by a BS through a higher layer message.

Time, frequency, and code resources used to send an ACK/NACK signal are called ACK/NACK resources or PUCCH resources. As described above, an index of ACK/NACK resources (called an ACK/NACK resource index or PUCCH index) used to send an ACK/NACK signal on a PUCCH can be represented as at least one of an orthogonal sequence index i, a CS index I_(cs), an RB index m, and an index for calculating the 3 indices. ACK/NACK resources can include at least one of an orthogonal sequence, a CS, a resource block, and a combination of them.

FIG. 7 shows the channel structure of the PUCCH formats 2/2a/2b in a normal CP.

Referring to FIG. 7, in a normal CP, OFDM symbols 1 and 5 (i.e., second and sixth OFDM symbols) are used to send a demodulation reference signal (DM RS), that is, an uplink reference signal, and the remaining OFDM symbols are used to send a CQI. In the case of an extended CP, an OFDM symbol 3 (fourth symbol) is used for a DM RS.

10 CQI information bits can be subject to channel coding at a 1/2 code rate, for example, thus becoming 20 coded bits. Reed-Muller code can be used in the channel coding. Next, the 20 coded bits are scramble and then subject to QPSK constellation mapping, thereby generating a QPSK modulation symbol (d(0) to d(4) in a slot 0). Each QPSK modulation symbol is modulated in a cyclic shift of a base RS sequence ‘r(n)’ having a length of 12, subject to IFFT, and then transmitted in each of 10 SC-FDMA symbols within a subframe. Uniformly spaced 12 CSs enable 12 different MSs to be orthogonally multiplexed in the same PUCCH RB. A base RS sequence ‘r(n)’ having a length of 12 can be used as a DM RS sequence applied to OFDM symbols 1 and 5.

FIG. 8 shows an example of a channel structure of a PUCCH format 3.

Referring to FIG. 8, the PUCCH format 3 is a PUCCH format which uses a block spreading scheme. The block spreading scheme means a method of spreading a symbol sequence, which is obtained by modulating a multi-bit ACK/NACK, in a time domain by using a block spreading code.

In the PUCCH format 3, a symbol sequence (e.g., ACK/NACK symbol sequence) is transmitted by being spread in the time domain by using the block spreading code. An orthogonal cover code (OCC) may be used as the block spreading code. Control signals of several UEs may be multiplexed by the block spreading code. In the PUCCH format 2, a symbol (e.g., d(0), d(1), d(2), d(3), d(4), etc., of FIG. 7) transmitted in each data symbol is different, and UE multiplexing is performed using the cyclic shift of a constant amplitude zero auto-correlation (CAZAC) sequence. In contrast, in the PUCCH format 3, a symbol sequence including one or more symbols is transmitted in a frequency domain of each data symbol, the symbol sequence is spread in a time domain by using the block spreading code, and UE multiplexing is performed. An example in which 2 RS symbols are used in one slot has been illustrated in FIG. 8, but the present invention is not limited thereto. 3 RS symbols may be used, and an OCC having a spreading factor value of 4 may be used. An RS symbol may be generated from a CAZAC sequence having a specific cyclic shift and may be transmitted in such a manner that a plurality of RS symbols in the time domain has been multiplied by a specific OCC.

Now, a carrier aggregation system is described. The carrier aggregation system is also called a multiple carrier system.

A 3GPP LTE system supports a case where a DL bandwidth and a UL bandwidth are differently configured, but one component carrier (CC) is a precondition in this case. A 3GPP LTE system supports a maximum of 20 MHz and may be different in a UL bandwidth and a DL bandwidth, but supports only one CC in each of UL and DL

A carrier aggregation (also called a bandwidth aggregation or a spectrum aggregation) supports a plurality of CCs. For example, if 5 CCs are allocated as the granularity of a carrier unit having a 20 MHz bandwidth, a maximum of a 100 MHz bandwidth may be supported.

FIG. 9 shows an example of a comparison between a single carrier system and a carrier aggregation system.

A carrier aggregation system (FIG. 9 (b)) has been illustrated as including three DL CCs and three UL CCs, but the number of DL CCs and UL CCs is not limited. A PDCCH and a PDSCH may be independently transmitted in each DL CC, and a PUCCH and a PUSCH may be independently transmitted in each UL CC. Or, a PUCCH may be transmitted only through a specific UL CC.

Since three pairs of DL CCs and UL CCs are defined, it can be said that a UE is served from three serving cells. Hereinafter, a cell which is configured to provide a service to a user equipment is referred to a serving cell.

The UE may monitor PDCCHs in a plurality of DL CCs and receive DL transport blocks through the plurality of DL CCs at the same time. The UE may send a plurality of UL transport blocks through a plurality of UL CCs at the same time.

A pair of a DL CC #A and a UL CC #A may become a first serving cell, a pair of a DL CC #B and a UL CC #B may become a second serving cell, and a DL CC #C and a UL CC#C may become a third serving cell. Each serving cell may be identified by a cell index (CI). The CI may be unique within a cell or may be UE-specific.

The serving cell may be divided into a primary cell and a secondary cell. The primary cell is a cell on which the UE performs an initial connection establishment procedure or initiates a connection re-establishment procedure, or a cell designated as a primary cell in a handover process. The primary cell is also called a reference cell. The secondary cell may be configured after an RRC connection has been established and may be used to provide additional radio resources. At least one primary cell is always configured, and a secondary cell may be added/modified/released in response to higher layer signaling (e.g., an RRC message). The CI of the primary cell may be fixed. For example, the lowest CI may be designated as the CI of the primary cell.

The primary cell includes a downlink primary component carrier (DL PCC) and an uplink PCC (UL PCC) in view of a CC. The secondary cell includes only a downlink secondary component carrier (DL SCC) or a pair of a DL SCC and a UL SCC in view of a CC. Hereinafter, the term, ‘cell’ may be mixed with the term ‘component carrier (CC)’.

As described above, the carrier aggregation system may support a plurality of CCs, that is, a plurality of serving cells unlike the single carrier system.

Such a carrier aggregation system may support cross-carrier scheduling. The cross-carrier scheduling is a scheduling method capable of performing resource allocation of a PDSCH transmitted through a different component carrier through a PDCCH transmitted through a specific component carrier and/or resource allocation of a PUSCH transmitted through other component carriers except for a component carrier fundamentally linked with the specific component carrier. That is, the PDCCH and the PDSCH may be transmitted through different DL CCs, and a PUSCH may be transmitted through a UL CC different from a UL CC linked with a DL CC to which a PDCCH including a UL is transmitted. As described above, in a system for supporting the cross-carrier scheduling, the PDCCH needs a carrier indicator indicating that PDSCH/PUSCH are transmitted through a certain DL CC/UL CC. Hereinafter, a field including the carrier indicator refers to a carrier indication field (CIF).

The carrier aggregation system that supports the cross-carrier scheduling may include a carrier indication field (CIF) to the conventional downlink control information (DCI). In a system that supports the cross-carrier scheduling, for example, LTE-A system, 3 bits may be extended since the CIF is added to the conventional DCI format (i.e., the DCI format used in LTE), and the PDCCH structure may reuse the conventional coding method, resource allocation method (i.e., resource mapping based on the CCE), and the like.

A BS may set a PDCCH monitoring DL CC (monitoring CC) group. The PDCCH monitoring DL CC group is configured by a part of all aggregated DL CCs. If the cross-carrier scheduling is configured, the UE performs PDCCH monitoring/decoding for only a DL CC included in the PDCCH monitoring DL CC group. That is, the BS transmits a PDCCH with respect to a PDSCH/PUSCH to be scheduled through only the DL CCs included in the PDCCH monitoring DL CC group. The PDCCH monitoring DL CC group may be configured in a UE-specific, UE group-specific, or cell-specific manner.

Non-cross carrier scheduling (NCSS) is a scheduling method capable of performing resource allocation of a PDSCH transmitted through a specific component carrier through a PDCCH transmitted through the specific component carrier and/or resource allocation of a PDSCH transmitted through a component carrier fundamentally linked with the specific component carrier.

ACK/NACK transmission for HARQ in 3GPP LTE Time Division Duplex (TDD) is described below.

In TDD, unlike in a Frequency Division Duplex (FDD), a DL subframe and an UL subframe coexist in one radio frame. In general, the number of UL subframes is smaller than that of DL subframes. Accordingly, in preparation for a case where UL subframes for sending an ACK/NACK signal are not sufficient, a plurality of ACK/NACK signals for DL transport blocks received in a plurality of DL subframes is transmitted in one UL subframe.

In accordance with section 10.1 of 3GPP TS 36.213 V8.7.0 (2009 May), two ACK/NACK modes: ACK/NACK bundling and ACK/NACK multiplexing are initiated.

In ACK/NACK bundling, UE sends ACK if it has successfully decoded all received PDSCHs (i.e., DL transport blocks) and sends NACK in other cases. To this end, ACK or NACKs for each PDSCH are compressed through logical AND operations.

ACK/NACK multiplexing is also called ACK/NACK channel selection (or simply channel selection). In accordance with ACK/NACK multiplexing, UE selects one of a plurality of PUCCH resources and sends ACK/NACK.

Table below shows DL subframes n-k associated with an UL subframe n according to an UL-DL configuration in 3GPP LTE, wherein k∈K and M is the number of elements of a set K.

TABLE 5 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 — — 6 — 4 — — 6 — 4 1 — — 7, 6 4 — — — 7, 6 4 — 2 — — 8, 7, 4, — — — — 8, 7, 4, — — 6 6 3 — — 7, 6, 11 6, 5 5, 4 — — — — — 4 — — 12, 8, 7, 6, 5, 4, — — — — — — 11 7 5 — — 13, 12, — — — — — — — 9, 8, 7, 5, 4, 11, 6 6 — — 7 7 5 — — 7 7 —

It is assumed that M DL subframes are associated with the UL subframe n and, for example, M=3. In this case, UE can obtain 3 PUCCH resources n⁽¹⁾ _(PUCCH,0), n⁽¹⁾ _(PUCCH,1), and n⁽¹⁾ _(PUCCH,2) because it can receive 3 PDCCHs from 3 DL subframes. In this case, an example of ACK/NACK channel selection is the same as the following table.

TABLE 6 HARQ-ACK(0), HARQ-ACK(1), HARQ-ACK(2) n⁽¹⁾ _(PUCCH) b(0), b(1) ACK, ACK, ACK n⁽¹⁾ _(PUCCH, 2) 1, 1 ACK, ACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1) 1, 1 ACK, NACK/DTX, ACK n⁽¹⁾ _(PUCCH, 0) 1, 1 ACK, NACK/DTX, NACK/DTX n⁽¹⁾ _(PUCCH, 0) 0, 1 NACK/DTX, ACK, ACK n⁽¹⁾ _(PUCCH, 2) 1, 0 NACK/DTX, ACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1) 0, 0 NACK/DTX, NACK/DTX, ACK n⁽¹⁾ _(PUCCH, 2) 0, 0 DTX, DTX, NACK n⁽¹⁾ _(PUCCH, 2) 0, 1 DTX, NACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1) 1, 0 NACK, NACK/DTX, NACK/DTX n⁽¹⁾ _(PUCCH, 0) 1, 0 DTX, DTX, DTX N/A N/A

In the above table, HARQ-ACK(i) indicates ACK/NACK for an i^(th) DL subframe of M DL subframes. Discontinuous transmission (DTX) means that a DL transport block has not been received on a PDSCH in a corresponding DL subframe or that a corresponding PDCCH has not been detected. In accordance with Table 6, 3 PUCCH resources n⁽¹⁾ _(PUCCH,0), n⁽¹⁾ _(PUCCH,1), and n⁽¹⁾ _(PUCCH,2) are present, and b(0), b(1) are two bits transmitted using a selected PUCCH.

For example, when UE successfully receives all 3 DL transport blocks in 3 DL subframes, the UE performs QPSK modulation on bits (1,1) using n⁽¹⁾ _(PUCCH,2) and sends them on a PUCCH. If UE fails in decoding a DL transport block in a first (i=0) DL subframe, but succeeds in decoding the remaining transport blocks, the UE sends bits (1,0) on a PUCCH using n⁽¹⁾ _(PUCCH,2). That is, in the existing PUCCH format 1b, only ACK/NACK of 2 bits can be transmitted. However, in channel selection, allocated PUCCH resources are linked to an actual ACK/NACK signal in order to indicate more ACK/NACK states. This channel selection is also referred to as channel selection using the PUCCH format 1b.

In ACK/NACK channel selection, if at least one ACK is present, NACK and DTX are coupled. This is because all ACK/NACK states cannot be represented by a combination of reserved PUCCH resources and a QPSK symbol. If ACK is not present, however, DTX is decoupled from NACK.

The above-described ACK/NACK bundling and ACK/NACK multiplexing can be applied in the case where one serving cell has been configured in UE in TDD.

For example, it is assumed that one serving cell has been configured (i.e., only a primary cell is configured) in UE in TDD, ACK/NACK bundling or ACK/NACK multiplexing is used, and M=1. That is, it is assumed that one DL subframe is associated with one UL subframe.

1) UE sends ACK/NACK in a subframe n if the UE detects a PDSCH indicated by a corresponding PDCCH in a subframe n-k of a primary cell or detects a Semi-Persistent Scheduling (SPS) release PDCCH. In LTE, a BS can inform UE that semi-persistent transmission and reception are performed in what subframes through a higher layer signal, such as Radio Resource Control (RRC). Parameters given by the higher layer signal can be, for example, the periodicity of a subframe and an offset value. When the UE receives the activation or release signal of SPS transmission through a PDCCH after recognizing semi-persistent transmission through the RRC signaling, the UE performs or releases SPS PDSCH reception or SPS PUSCH transmission. That is, the UE does not immediately perform SPS transmission/reception although SPS scheduling is allocated thereto through the RRC signaling, but when an activation or release signal is received through a PDCCH, performs SPS transmission/reception in a subframe that corresponds to frequency resources (resource block) according to the allocation of the resource block designated by the PDCCH, modulation according to MCS information, a subframe periodicity allocated through the RRC signaling according to a code rate, and an offset value. Here, a PDCCH that releases SPS is called an SPS release PDCCH, and a DL SPS release PDCCH that releases DL SPS transmission requires the transmission of an ACK/NACK signal.

Here, in the subframe n, UE sends ACK/NACK using the PUCCH formats 1a/1b according to a PUCCH resource n^((1,p)) _(PUCCH). In n^((1,p)) _(PUCCH), p indicates an antenna port p. The k is determined by Table 5.

The PUCCH resource n^((1,p)) _(PUCCH) can be allocated as in the following equation. P can be p0 or p1.

n ^((1,p=p0) _(PUCCH)=(M−m−1)·N _(c) +m·N _(c+1) +n _(CCE) +N ⁽¹⁾ _(PUCCH) for antenna port p=p0,

n ^((1,p=p1)) _(PUCCH)=(M−m−1)·N _(c) +m·N _(c+1)+(n _(CCE)+1)+N ⁽¹⁾ _(PUCCH) for antenna port p=p1,  [Equation 3])

In Equation 3, c is selected in such a way as to satisfy N_(c)≤n_(CCE)<N_(c+1) (antenna port p0), N_(c)≤(n_(CCE)+1)<N_(c+1) (antenna port p1) from among {0, 1, 2, 3}. N⁽¹⁾ _(PUCCH) is a value set by a higher layer signal. N_(C)=max{0, floor [N^(DL) _(RB)·(N^(RB) _(sc)·c−4)/36]}. The N^(DL) _(RB) is a DL bandwidth, and N^(RB) _(sc) is the size of an RB indicated by the number of subcarriers in the frequency domain. n_(CCE) is a first CCE number used to send a corresponding PDCCH in a subframe n−km. m is a value that makes km the smallest value in the set K of Table 5.

2) If UE detects an SPS PDSCH, that is, a PDSCH not including a corresponding PDCCH, in the DL subframe n-k of a primary cell, the UE can send ACK/NACK in the subframe n using the PUCCH resource n^((1,p)) _(PUCCH) as follows.

Since an SPS PDSCH does not include a scheduling PDCCH, UE sends ACK/NACK through the PUCCH formats 1a/1b according to n^((1,p)) _(PUCCH) that is configured by a higher layer signal. For example, 4 resources (a first PUCCH resource, a second PUCCH resource, a third PUCCH resource, and a fourth PUCCH resource) can be reserved through an RRC signal, and one resource can be indicated through the Transmission Power Control (TPC) field of a PDCCH that activates SPS scheduling.

The following table is an example in which resources for channel selection are indicated by a TPC field value.

TABLE 7 TPC field value Resource for channel selection ‘00’ First PUCCH resource ‘01’ Second PUCCH resource ‘10’ Third PUCCH resource ‘11’ Fourth PUCCH resource

For another example, it is assumed that in TDD, one serving cell is configured (i.e., only a primary cell is configured) in UE, ACK/NACK multiplexing is used, and M>1. That is, it is assumed that a plurality of DL subframes is associated with one UL subframe.

1) A PUCCH resource n⁽¹⁾ _(PUCCH,i) for sending ACK/NACK when UE receives a PDSCH in a subframe n−k_(i) (0≤i≤M−1) or detects a DL SPS release PDCCH can be allocated as in the following equation. Here, k_(i)∈K, and the set K has been described with reference to Table 5.

n ⁽¹⁾ _(PUCCH,i)=(M−i−1)·N _(c) +i·N _(c+1) +n _(CCE,i) +N ⁽¹⁾ _(PUCCH)  [Equation 4]

Here, c is selected from {0, 1, 2, 3} so that N_(c)≤n_(CCE,i)<N_(c+1) is satisfied. N⁽¹⁾ _(PUCCH) is a value set by a higher layer signal. N_(C)=max{0, floor [N^(DL) _(RB)·(N^(RB) _(sc)·c−4)/36]}. The N^(DL) _(RB) is a DL bandwidth, and N^(RB) _(sc) is the size of an RB indicated by the number of subcarriers in the frequency domain. n_(CCE,i) is a first CCE number used to send a corresponding PDCCH in the subframe n−k_(i).

2) If UE receives a PDSCH (i.e., SPS PDSCH) not having a corresponding PDCCH in the subframe, n⁽¹⁾ _(PUCCH,i) is determined by a configuration given by a higher layer signal and Table 7.

If two or more serving cells have been configured in UE in TDD, the UE sends ACK/NACK using channel selection that uses the PUCCH format 1b or the PUCCH format 3. Channel selection that uses the PUCCH format 1b used in TDD can be performed as follows.

If a plurality of serving cells using channel selection that uses the PUCCH format 1b has been configured, when ACK/NACK bits are greater than 4 bits, UE performs spatial ACK/NACK bundling on a plurality of codewords within one DL subframe and sends spatially bundled ACK/NACK bits for each serving cell through channel selection that uses the PUCCH format 1b. Spatial ACK/NACK bundling means the compression of ACK/NACK for each codeword through logical AND operations within the same DL subframe.

If ACK/NACK bits are 4 bits or lower, spatial ACK/NACK bundling is not used and the ACK/NACK bits are transmitted through channel selection that uses the PUCCH format 1b.

If 2 or more serving cells using the PUCCH format 3 have been configured in UE, when ACK/NACK bits are greater than 20 bits, spatial ACK/NACK bundling can be performed in each serving cell and ACK/NACK bits subjected to spatial ACK/NACK bundling can be transmitted through the PUCCH format 3. If ACK/NACK bits are 20 bits or lower, spatial ACK/NACK bundling is not used and the ACK/NACK bits are transmitted through the PUCCH format 3.

<Channel Selection Using the PUCCH Format 1b Used in FDD>

If two serving cells using FDD have been configured in UE, ACK/NACK can be transmitted through channel selection that uses the PUCCH format 1b. The UE can feed ACK/NACK for a maximum of 2 transport blocks, received in one serving cell, back to a BS by sending 2-bit (b(0)b(1)) information in one PUCCH resource selected from a plurality of PUCCH resources. One codeword can be transmitted in one transport block. A PUCCH resource can be indicated by a resource index n⁽¹⁾ _(PUCCH,i). Here, A is any one of {2, 3, 4}, and i is 0≤i≤(A−1). The 2-bit information is indicated as b(0)b(1).

HARQ-ACK(j) indicates an HARQ ACK/NACK response that is related to a transport block or DL SPS release PDCCH transmitted by a serving cell. The HARQ-ACK(j), the serving cell, and the transport block can have the following mapping relationship.

TABLE 8 HARQ-ACK(j) HARQ- HARQ- HARQ- HARQ- A ACK(0) ACK(1) ACK(2) ACK(3) 2 Transport Transport NA NA block 1 of block 2 of primary cell secondary cell 3 Transport Transport Transport NA block 1 of block 2 of block 3 of serving cell 1 serving cell 1 serving cell 2 4 Transport Transport Transport Transport block 1 of block 2 of block 3 of block 4 of primary cell primary cell secondary cell secondary cell

In Table 8, for example, in the case of A=4, HARQ-ACK(0) and HARQ-ACK(1) indicate ACK/NACKs for 2 transport blocks transmitted in a primary cell, and HARQ-ACK(2) and HARQ-ACK(3) indicate ACK/NACKs for 2 transport blocks transmitted in a secondary cell.

When UE receives a PDSCH or detects a DL SPS release PDCCH by detecting a PDCCH in a subframe ‘n−4’ of a primary cell, the UE sends ACK/NACK using a PUCCH resource n⁽¹⁾ _(PUCCH,i). Here, n⁽¹⁾ _(PUCCH,i) is determined to be n_(CCE,i)+N⁽¹⁾ _(PUCCH). Here, n_(CCE,i) means an index of the first CCE that is used to send a PDCCH by a BS, and N⁽¹⁾ _(PUCCH) is a value set through a higher layer signal. If a transmission mode of a primary cell supports up to two transport blocks, a PUCCH resource n⁽¹⁾ _(PUCCH,i+1) is given. Here, n⁽¹⁾ _(PUCCH,i+1) can be determined to be n_(CCE,i)+1+N⁽¹⁾ _(PUCCH). That is, if a primary cell is set in a transmission mode in which a maximum of up to 2 transport blocks can be transmitted, 2 PUCCH resources can be determined.

If a PDCCH detected in a subframe ‘n−4’ of a primary cell is not present, a PUCCH resources n⁽¹⁾ _(PUCCH,i) for sending ACK/NACK for a PDSCH is determined by a higher layer configuration. If up to 2 transport blocks are supported, a PUCCH resource n⁽¹⁾ _(PUCCH,i+1) can be given as n⁽¹⁾ _(PUCCH,i+1)=n⁽¹⁾ _(PUCCH,i+1).

If a PDSCH is received in a secondary cell by detecting a PDCCH in a subframe ‘n−4’, PUCCH resources n⁽¹⁾ _(PUCCH,i) and n⁽¹⁾ _(PUCCH,i+1) for a transmission mode in which up to 2 transport blocks are supported can be determined by a higher layer configuration.

Meanwhile, in the prior art, it was a precondition that a plurality of serving cells configured in a UE uses radio frames having the same type. For example, it was a precondition that all of a plurality of serving cells configured in the UE use FDD frames or use TDD frames. In the next-generation wireless communication system, however, different types of radio frames may be used respectively in serving cells.

FIG. 10 shows one example in which a plurality of serving cells use different types of radio frames in a wireless communication system.

Referring to FIG. 10, a primary cell PCell and a plurality of secondary cells SCell #1, . . . , SCell #N may be configured in a UE. In this case, the primary cell may operate in FDD and use an FDD frame, and the secondary cells may operate in TDD and use TDD frames. The same UL-DL configuration may be used in the plurality of secondary cells. A DL subframe (indicated by D) and a UL subframe (indicated by U) are present in a 1:1 manner in the primary cell, but a DL subframe and a UL subframe may be present in a different ratio other than 1:1 in the secondary cells.

Table 9 below shows that ACK/NACK is transmitted in what a subframe according to a UL-DL configuration when one serving cell operates in TDD. Table 9 is equivalent to Table 5.

TABLE 9 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 4 6 — 4 6 — 1 7 6 4 7 6 4 2 7 6 4 8 7 6 4 8 3 4 11  7 6 6 5 5 4 12  11  8 7 7 6 5 4 5 12  11  9 8 7 6 5 4 13  6 7 7 7 7 5

In Table 9, when a UE receives a PDSCH or a PDCCH (e.g., DL SPS release PDCCH) necessary for an ACK/NACK response in a subframe n, the UE sends ACK/NACK in a subframe n+k(n). Each of the values of Table 9 indicates the k(n) value. For example, Table 9 indicates that if a UL-DL configuration is 0 and a PDSCH is received in a subframe 0, ACK/NACK is transmitted after four subframes elapse, i.e., in a subframe 4. A specific time is necessary in order for the UE to send ACK/NACK after receiving a PDSCH or a DL SPS release PDCCH. A minimum value of this specific time is hereinafter indicated as k_(min), and a value of k_(min) may be four subframes. Four subframes, which is the minimum value of the specific time, are determined by considering a propagation delay between the transmission terminal and the reception terminal, a processing time which is required for decoding at the reception terminal. When looking at the timing of transmitting ACK/NACK in Table 9 above, the ACK/NACK is mainly transmitted in the first uplink subframe after a lapse of k_(min). However, the underlined figures do not indicate the first uplink subframe after a lapse of k_(min), but indicate the uplink subframe which is located at the next position. The reason for this is to prevent from transmitting the ACK/NACK for too many downlink subframes in one uplink subframe.

Meanwhile, since a UL subframe: DL subframe ratio is always 1:1 in FDD, ACK/NACK timing is determined as shown in the following table.

TABLE 9-1 Frame Subframe n Structure 0 1 2 3 4 5 6 7 8 9 FDD 4 4 4 4 4 4 4 4 4 4

That is, as shown in the above Table, k(n)=k_(min)=4 for all subframes.

Meanwhile, in the prior art, it was a precondition that all serving cells use radio frames having the same type, and ACK/NACK transmission timing, that is, HARQ timing, was determined based on this assumption. However, if a plurality of serving cells use different types of radio frames, it is necessary to determine which method will be used to transmit ACK/NACK.

It is hereinafter assumed that a primary cell and at least one secondary cell are configured in a UE in a wireless communication system. It is also assumed that the primary cell uses an FDD frame and the secondary cell uses a TDD frame. Any one of the UL-DL configurations of Table 1 may be used in the TDD frame. Hereinafter, only a relationship between a primary cell and one secondary cell is illustrated, for convenience of description, but this relationship may be applied to a relationship between a primary cell and each of a plurality of secondary cells when the plurality of secondary cells are configured in the UE.

Under this assumption, first, a method of transmitting ACK/NACK for downlink data received through a primary cell is described below. Hereinafter, the downlink data generally indicates a PDSCH that requests an ACK/NACK response, a codeword included in a PDSCH, a DL SPS release PDCCH indicating a DL SPS release and the like.

FIG. 11 shows a method of transmitting ACK/NACK for downlink data received through a primary cell.

Referring to FIG. 11, a BS sends downlink data in a subframe n of a primary cell (S110). From a view of a UE, the downlink data is received in a subframe n of a DL PCC of the primary cell.

The UE decodes the downlink data and generates ACK/NACK for the downlink data (S120).

The UE sends the ACK/NACK in a subframe n+k_(PCC)(n) of the primary cell (S130).

The subframe n+k_(PCC)(n) of the primary cell is a subframe after a minimum delay time (this is called k_(min)) necessary for an ACK/NACK response has elapsed from a point of time at which the downlink data was received. Here, the minimum delay time k_(min) may be four subframes. Accordingly, the UE may send the ACK/NACK in a subframe n+4 of a UL PCC of the primary cell.

That is, in the primary cell, as in the case where an HARQ is performed in conventional FDD, the ACK/NACK is transmitted in a subframe after four subframes elapse from a subframe in which data was received.

Now, a method of sending ACK/NACK when a UE receives downlink data in a secondary cell is described.

FIG. 12 shows a method of transmitting ACK/NACK for downlink data received through a secondary cell.

Referring to FIG. 12, a BS sends information about a UL-DL configuration of the secondary cell (S210). The secondary cell may need the UL-DL configuration information because it operates in TDD. The UL-DL configuration information may be transmitted through a higher layer signal, such as an RRC message.

A BS sends downlink data in a subframe n of the secondary cell (S220).

The UE decodes the downlink data and generates ACK/NACK for the downlink data (S230).

The UE may send the ACK/NACK to the BS through a subframe n+k_(SCC)(n) of a primary cell (S240). The subframe n+k_(SCC)(n) may be determined by the following method.

<HARQ ACK/NACK Transmission Timing in System in which CCs Using Different Frame Structures are Aggregated>

<Method 1>

Method 1 is a method in which a subframe n+k_(SCC)(n) complies with ACK/NACK transmission timing in a primary cell. That is, Method 1 is a method of configuring a UL subframe of the primary cell equal to n+k_(min) as the subframe n+k_(SCC)(n). In other words, if data is received in a subframe n of a secondary cell, ACK/NACK for the data is transmitted in the subframe n+k_(min) of the primary cell. Here, k_(min) may be, for example, four subframes.

FIG. 13 shows an example of ACK/NACK transmission timing when a primary cell is an FDD cell and a secondary cell is a TDD cell.

Referring to FIG. 13, it is assumed that a UL subframe of PCC in which ACK/NACK is transmitted for a DL data channel or DL control channel received in a DL subframe n of PCC is a subframe n+k_(PCC)(n). In case of FDD, to avoid an ACK/NACK transmission delay, it may be set to k_(PCC)(n)=k_(min)=4 similarly in the conventional way.

It is assumed that a UL subframe of PCC in which ACK/NACK is transmitted for a DL data channel or DL control channel received in a DL subframe n of SCC is a subframe n+k_(SCC)(n). Then, k_(SCC)(n) may comply with ACK/NACK timing of FDD configured in PCC. That is, it may be set to k_(SCC)(n)=k_(min)=4. For example, ACK/NACK for a DL data channel or DL control channel received in a subframe n 131 of SCC is transmitted in a subframe n+4 132 of PCC.

FIG. 14 shows an ACK/NACK transmission method based on Method 1.

Referring to FIG. 14, in a situation where a 1^(st) cell and a 2^(nd) cell are aggregated, data requiring ACK/NACK is received in a DL subframe of the 2^(nd) cell (S161). Herein, the data requiring ACK/NACK collectively refers to data requiring an ACK/NACK response such as a PDSCH, a transport block, and a DL SPS release PDCCH. The 1^(st) cell is an FDD cell using an FDD frame, and may be a primary cell. The 2^(nd) cell is a TDD cell using a TDD frame, and may be a secondary cell.

A UE transmits ACK/NACK for the data in a UL subframe of the 1⁴ cell determined according to ACK/NACK timing of the 1⁴ cell (S162).

In accordance with Method 1, there is an advantage in that ACK/NACK delay is minimized because ACK/NACK for downlink data received in the secondary cell is always transmitted after k_(min) subframes elapse on the basis of a point of time at which the downlink data was received.

Furthermore, in conventional TDD, if the number of DL subframes associated with one UL subframe is many, there is a problem in that the number of ACK/NACKs that must be transmitted in the one UL subframe is increased. However, Method 1 is advantageous in that ACK/NACK transmission is distributed.

If the UL subframe of a primary cell in which ACK/NACK is transmitted is a subframe n, the number of ACK/NACK resources that need to be secured in the subframe n may be determined by a transmission mode of the primary cell for a subframe n−k_(min) and a transmission mode in a DL subframe of the secondary cell.

In accordance with Method 1, ACK/NACK timing applied to the UE may be represented by changing Table 5 into Table 10 below.

TABLE 10 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 4 — — — 4 4 — — — 4 1 4 — — 4 4 4 — — 4 4 2 4 — 4 4 4 4 — 4 4 4 3 4 4 4 4 4 4 — — — 4 4 4 4 4 4 4 4 — — 4 4 5 4 4 4 4 4 4 — 4 4 4 6 4 — 4 4 4 — 4

That is, if a UL-DL configuration of the secondary cell is the same as any one of Table 10 and the primary cell uses an FDD frame, a subframe n is a subframe in which ACK/NACK is transmitted and a number indicated in the subframe n indicates k_(min). Herein, the subframe n−k_(min) indicates a subframe in which downlink data, that is, the subject of ACK/NACK, is received. For example, in Table 10, a UL-DL configuration is 0, and 4 is written in a subframe 9. In this case, it indicates that ACK/NACK for downlink data received in the subframe 5 (=9−4) of the secondary cell is transmitted in the subframe 9.

In accordance with Method 1, ACK/NACK timing applied to the UE may be represented by changing Table 9 into Table 11 below.

TABLE 11 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 4 4 — 4 4 — 1 4 4 4 4 4 4 2 4 4 4 4 4 4 4 4 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 4 4 4 4 4 4 4 4 4 6 4 4 4 4 4

In Table 11, a subframe n indicates a subframe in which downlink data is received. A subframe n+k_(SCC)(n) is a subframe in which ACK/NACK for the downlink data is transmitted. Each of values in Table 11 indicates a k_(SCC)(n) value for the subframe n. For example, it indicates that, if a UL-DL configuration is 0 and downlink data is received in the subframe 1 of a secondary cell, ACK/NACK is transmitted in a subframe 5 (of a primary cell) after four subframes elapse.

Tables 10 and 11 and FIG. 13 have a precondition that the radio frame boundaries of a secondary cell and a primary cell are the same. That is, it is a precondition that the radio frame of the primary cell is synchronized with the radio frame of the secondary cell. If the radio frame of the primary cell is not synchronized with the radio frame of the secondary cell, additional subframe delay (indicated by k_(add)) for compensating for this asynchronization may be taken into consideration. That is, in Method 1, k_(SCC)(n) may be changed into k_(min)+k_(add).

Or, assuming that downlink data is received in the subframe n of a secondary cell and a subframe in which ACK/NACK for the downlink data is transmitted is n+k_(SCC)(n), if the k_(SCC)(n) is smaller than k_(min)+k_(add), scheduling may be limited so that the downlink data is not transmitted in the subframe n of the secondary cell.

<Method 2>

Method 2 is a method of determining a subframe n+k_(SCC)(n) in which ACK/NACK is transmitted based on TDD ACK/NACK transmission timing in a secondary cell. That is, k_(SCC)(n) is determined as in Table 9, but actual ACK/NACK is transmitted through the UL PCC of a primary cell. In other words, ACK/NACK for a DL data channel or DL control channel received in SCC may be transmitted in a UL subframe of PCC according to ACK/NACK timing configured in SCC.

FIG. 15 shows another example of ACK/NACK transmission timing when a primary cell is an FDD cell and a secondary cell is a TDD cell.

Referring to FIG. 15, it is assumed that a UL subframe of PCC in which ACK/NACK is transmitted for a DL data channel or DL control channel received in a DL subframe n of PCC is a subframe n+k_(SCC)(n). In case of FDD, to avoid an ACK/NACK transmission delay, it may be set to k_(PCC)(n)=k_(min)=4 similarly in the conventional way.

In this case, ACK/NACK for a DL data channel or DL control channel received in a DL subframe n 141 of SCC may be transmitted in a UL subframe n+k(n) 142 of SCC when ACK/NACK timing configured in SCC is applied. In this case, the ACK/NACK is transmitted in a UL subframe 143 of PCC at a time equal to that of the UL subframe n+k(n) 142.

FIG. 16 shows an ACK/NACK transmission method based on Method 2.

Referring to FIG. 16, in a situation where a 1^(st) cell and a 2^(nd) cell are aggregated, data requiring ACK/NACK is received in a DL subframe of the 2^(nd) cell (S151). Herein, the data requiring ACK/NACK refers to data requiring an ACK/NACK response such as a PDSCH, a transport block, and a DL SPS release PDCCH. The 1^(st) cell is an FDD cell using an FDD frame, and may be a primary cell. The 2^(nd) cell is a TDD cell using a TDD frame, and may be a secondary cell.

A UE transmits ACK/NACK for the data in a UL subframe of the 1^(st) cell determined according to ACK/NACK timing applied when only the 2^(nd) cell is configured (S152).

Such a method has an advantage in that ACK/NACK timing for TDD CC may be equally applied irrespective of whether the TDD CC is used as a primary cell or secondary cell.

The number of resources for ACK/NACK transmission, which must be ensured in a UL subframe of PCC, is determined according to whether a DL subframe is present in a PCC/SCC at a subframe n and according to a transmission mode at the present DL subframe.

If the radio frame of a primary cell is not synchronized with that of a secondary cell, additional subframe delay (indicated by k_(add)) for compensating for this asynchronization may be taken into consideration. The k_(add) may be a fixed value or may be a value set through an RRC message. In Method 2, assuming that k′_(SCC)(n)=k_(SCC)(n) k_(add), ACK/NACK for downlink data received in the subframe n of the secondary cell may be represented as being transmitting in the UL subframe n+k′_(SCC)(n) of the primary cell.

Or, assuming that downlink data is received in the subframe n of the secondary cell and a subframe in which ACK/NACK for the downlink data is transmitted is n+k_(SCC)(n), if the k_(SCC)(n) is smaller than k_(min)+k_(add), scheduling may be limited so that the downlink data is not transmitted in the subframe n of the secondary cell.

If Method 1 is used as the method of transmitting ACK/NACK in a primary cell and the method of transmitting ACK/NACK for a secondary cell, the ACK/NACK for the primary cell and the secondary cell may comply with an ACK/NACK transmission scheme used in FDD. For example, channel selection may be used in which the PUCCH format 1b used in FDD is used when a plurality of serving cells are configured in a UE. That is, ACK/NACK for the secondary cell is transmitted using channel selection that uses the PUCCH format 1b through the primary cell without using a compression scheme, such as ACK/NACK bundling. A compression scheme, such as ACK/NACK bundling, may not be used because only one DL subframe is associated with one UL subframe of the primary cell.

In contrast, if Method 2 is used as the method of transmitting ACK/NACK in a primary cell and the method of transmitting ACK/NACK for a secondary cell, the ACK/NACK for the primary cell and the secondary cell may comply with an ACK/NACK transmission scheme used in TDD. For example, ACK/NACK may be transmitted through channel selection that uses the PUCCH format 1b used when a plurality of serving cells are configured in TDD.

Whether to apply the aforementioned Methods 1 and 2 may be determined according to whether to use cross carrier scheduling or non-cross carrier scheduling. For example, Method 1 may be used in the cross carrier scheduling and Method 2 may be used in the non-cross carrier scheduling.

If CCs to be aggregated use different frame structures (an aggregation of an FDD CC and a TDD CC), one CC may perform UL transmission and another CC may perform DL reception in the same time duration (or subframe). In this case, the UL transmission may have an effect on the DL reception. Therefore, it is not desirable to perform the UL transmission and the DL reception simultaneously in contiguous frequency bands.

To solve this problem, preferably, frequency bands separated enough not to be interfered from each other are grouped, so that the same UL-DL configuration is used in one group and different UL-DL configurations are used in different groups.

For example, if CCs #1 to #5 are aggregated in an ascending order of an allocated frequency band, the CCs #1 and #2 are grouped as a first group and the CCs #3 to #5 are grouped as a second group, and all CCs in the first group use a UL-DL configuration 0, and all CCs in the second group use a UL-DL configuration 3. In this case, the CC #2 and the CC #3 may be CCs separated enough not to be interfered from each other. In the above example, a UE may have an independent RF module for each group, and may use a separate power amplifier. The UE may transmit one PUCCH for each group, and in this case, a problem of a peak to average ratio (PAPR) increase does not occur even if a plurality of PUCCHs are transmitted in uplink.

If the PUCCH is transmitted only with PCC, Method 1 may be applied, and if the PUCCH is transmitted in a specific UL CC of a group (of a non-contiguous frequency band) to which the PCC does not belong, ACK/NACK timing transmitted through the PUCCH may comply with ACK/NACK timing corresponding to a DL subframe of the specific UL CC in which the PUCCH is transmitted.

<A Method for Determining the Number of DL HARQ Processes in Aggregation of Serving Cells that Use Different Frame Structures with Each Other>

FIG. 17 illustrates a DL HARQ that is performed in a cell of 3GPP LTE.

Referring to FIG. 17, a base station transmits a DL transmission block in subframe n onto a PDSCH 412 that is indicated by a DL resource allocation on a PDCCH 411 to a UE.

The UE transmits an ACK/NACK signal onto a PUCCH 420 in subframe n+4. As an example, the resource of the PUCCH 420 that is used for the transmission of ACK/NACK signal may be determined based on the resource of the PDCCH 411 (for example, the first CCE index that is used for the transmission of the PDCCH 411).

Even though the base station receives a NACK signal from the UE, it does not necessarily retransmit it in subframe n+8 unlike the UL HARQ. Here, an example is illustrated that a retransmission block is transmitted onto a PDSCH 432 that is indicated by the DL resource allocation on a PDCCH 431 in subframe n+9.

The UE transmits an ACK/NACK signal onto a PUCCH 440 in subframe n+13.

The following table shows a location of the shortest DL subframe after subframe 4 in each UL subframe in a TDD frame according to each of the UL-DL configurations. When a UE transmits an ACK/NACK in the UL subframe, a base station is required to perform scheduling based on this, which needs a certain amount of time, and the minimum value of this time is four subframes.

TABLE 12 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 4 7 6 4 7 6 1 4 6 4 6 2 4 4 3 4 4 4 4 4 4 5 4 6 4 6 5 4 7 FDD 4 4 4 4 4 4 4 4 4 4

For example, according to UL-DL configuration 0, subframes 2, 3, 4, 7, 8 and 9 are UL subframes, and the rest subframes are DL subframes (or special subframes). Based on UL-DL configuration 0 of the above table, subframe 4 indicates 6, and 6 represents that a shortest DL subframe located after four subframes is a 6^(th) subframe after subframe 4 (i.e., subframe 0 of the next frame) based on subframe 4. Likewise, in UL-DL configuration 6, subframe 8 indicates 7, and this represents that a shortest DL subframe located after four subframes is a 7^(th) subframe after subframe 8 (i.e., subframe 5 of the next frame) based on subframe 8.

That is, Table 12 above represents the shortest time to be taken by the base station to perform a transmission according to the ACK/NACK response (i.e., transmitting new data, or retransmitting the existing data) as a subframe unit, after the UE transmits the ACK/NACK in the DL HARQ.

Hereinafter, for the convenience of descriptions, the time of a DL HARQ process to be taken is defined as follows. First, what a base station transmits data refers to an initial transmission. The above data mentioned refers to all things that are required for an ACK/NACK response. For example, it does not limit to data such as a transmission block, codeword, PDSCH, etc., but includes a control channel that is required for the ACK/NACK response like a DL SPS release PDCCH.

The UE transmits an ACK/NACK for the above data. The base station may transmit new data or retransmit the data according to the ACK/NACK after receiving the ACK/NACK transmitted. The base station may transmit new data, however, let's refer to a retransmission to transmit data according to the ACK/NACK response for convenience sake.

Then, the time required for performing an identical DL HARQ process may be defined by the time difference between the initial transmission and the retransmission. The time required for performing the DL HARQ process may refer to the sum of the time to be taken till the initial transmission (it is marked to as k. Here, k is a unit of a subframe), which is an object of the ACK/NACK based on the UL subframe to which the ACK/NACK is transmitted, and the time to be taken till retransmission (it is marked to as j. Here, j is a unit of a subframe) after the ACK/NACK.

In the FDD, the proportion of DL subframe and UL subframe is 1:1 and the DL subframe and the UL subframes are sequentially existed in different frequency bandwidths within the FDD frame so that the time of DL HARQ process to be taken may be constant to be eight subframes.

Meanwhile, in the TDD, the k and j may be different for each UL subframe respectively so that the time required for performing DL HARQ process may not be constant.

The following table represents values of k+j in each of UL subframes within the TDD frame according to the UL-DL configurations. When a value k for the subframe n refers to k(n), and j(n) for a value j, the following Table 13 represents the subframe n in a form of {k(n)}+j(n). k(n) may be any one among the plurality of values.

TABLE 13 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 Max 0 — — {6} + 4 — {4} + 6 — — {6} + 4 — {4} + 6 10 1 — — {7, 6} + 4 {4} + 6 — — — {7, 6} + 4 {4} + 6 — 11 2 — — {8, 7, 4, 6} + 4 — — — — {8, 7, 4, 6} + 4 — — 12 3 — — {7, 6, 11} + 4 {6, 5} + 4 {5, 4} + 4 — — — — — 15 4 — — {12, 8, 7, 11} + 4 {6, 5, 4, 7} + 4 — — — — — — 16 5 — — {13, 12, 9, 8, 7, — — — — — — — 17 5, 4, 11, 6} + 4 6 — — {7} + 4 {7} + 6 {5} + 5 — — {7} + 4 {7} + 7 — 14 FDD {4} + 4 {4} + 4 {4} + 4 {4} + 4 {4} + 4 {4} + 4 {4} + 4 {4} + 4 {4} + 4 {4} + 4 8

In the Table 13, ‘Max’ represents the maximum value of the values of {k(n)}+j(n) that are obtained in each of the UL subframes according to the UL-DL configurations. For example, in UL-DL configuration 4, the maximum value of the values of {k(n)}+j(n) is 16.

Meanwhile, the number of DL HARQ processes that are operated in the DL is associated the maximum value of the values of {k(n)}+j(n). That is, the number of DL HARQ processes in a cell may be determined to be the number of DL subframes that are included within a section corresponding to the maximum value of the values of {k(n)}+j(n).

FIG. 18 illustrates an example of determining the number of DL HARQ processes in a FDD cell.

Referring to FIG. 18, k=4 and j=4, and the number of DL subframes that are included in a section of the subframe including the number of k+j is 8. Since each of the eight DL subframes may be used in different DL HARQ processes, the maximum number of DL HARQ processes (refer to M_(HARQ)) may be 8.

FIG. 19 illustrates an example of determining the number of DL HARQ processes for each of UL-DL configurations 0 to 2 in a TDD cell.

For example, according to UL-DL configuration 0, k is 6 and j is 4 based on a UL subframe 191. And the number of the DL subframes that are included in the section of k+j subframes is 4. Therefore, M_(HARQ) is 4 (M_(HARQ)=4).

FIG. 20 illustrates an example of determining the number of DL HARQ processes for each of UL-DL configurations 3 to 5 in a TDD cell.

For example, according to UL-DL configuration 3, k is 11 and j is 4 based on a UL subframe 201. And the number of the DL subframes that are included in the section of k+j subframes is 9. Therefore, M_(HARQ) is 9 (M_(HARQ)=9).

FIG. 21 illustrates an example of determining the number of DL HARQ processes for UL-DL configuration 6 a TDD cell.

For example, according to UL-DL configuration 6, k is 7 and j is 7 based on a UL subframe 211. And the number of the DL subframes that are included in the section of k+j subframes is 6. Therefore, M_(HARQ) is 6 (M_(HARQ)=6).

That is, FIGS. 18 to 21 represent examples of determining the number of DL HARQ processes for a cell.

Meanwhile, in case of applying method 1 or 2 described above, the ACK/NACK timing of a secondary cell may be different. For example, if method 1 is applied for the case that a primary cell is a FDD cell and a secondary cell is a TDD cell, the ACK/NACK for data received in the DL subframe of the secondary cell is transmitted to the UL subframe of the primary cell, and at the moment, the UL subframe depends on the ACK/NACK timing of the FDD cell that is a primary cell. Therefore, the both timings of the ACK/NACK timing in case that the TDD cell is exclusively configured and the ACK/NACK timing in case that cells that use different frame structures are aggregated may be different with each other. In this case, it may be problem how to determine the number of DL HARQ processes.

First of all, some technical terms are defined for the convenience of descriptions. Here, k_(s-p) represents a time difference between the DL subframe of the secondary cell that receives data and the UL subframe of the primary cell that transmits the ACK/NACK for the data. And, j_(p-s) represents a time difference between the UL subframe of the primary cell that transmits the ACK/NACK and the shortest DL subframe of the secondary cell that is available to be transmitted by an identical DL HARQ process.

If a primary cell is the FDD cell and a secondary cell is the TDD cell and the method 1 is used, j_(p-s) may be represented as follows for each subframe of the secondary cell.

TABLE 14 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 6/5 6 6/5 6 1 6/4 6 6/5 6/4 6 6/5 2 5/4 4 5 5/4 5/4 4 5 5/4 3 6/5 6/4 6/4 6/4 6/4 6/4 6 4 6/4 6/4 6/4 6/4 6/4 6/4 6 6/5 5 5/4 5/4 5/4 5/4 5/4 5/4 5 5/4 6 6/5 6 6/5 6/4 6

In the table, each subframe is represented as a form of A/B, here, A refers to a value for the case that a collision of DL HARQ processes is not allowed, and B refers to a value for the case that a collision of DL HARQ processes is allowed.

The following table represents k_(S-P)+j_(P-S) in the case that a collision of DL HARQ processes is not allowed for each subframe of the secondary cell. That is, if k_(S-P) for subframe n is k_(S-P)(n) and j_(P-S) for subframe n is j_(P-S) (n), the following table refers to k_(S-P)(n)+j_(P-S)(n) for subframe n.

TABLE 15 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 Max DL Min DL 0 4 + 6 — — — 4 + 6 4 + 6 — — — 4 + 6 4 4 1 4 + 6 — — 4 + 6 4 + 6 4 + 6 — — 4 + 6 4 + 6 6 6 2 4 + 5 — 4 + 5 4 + 5 4 + 5 4 + 5 — 4 + 5 4 + 5 4 + 5 8 7 3 4 + 6 4 + 6 4 + 6 4 + 6 4 + 6 4 + 6 — — — 4 + 6 7 7 4 4 + 6 4 + 6 4 + 6 4 + 6 4 + 6 4 + 6 — — 4 + 6 4 + 6 8 8 5 4 + 5 4 + 5 4 + 5 4 + 5 4 + 5 4 + 5 — 4 + 5 4 + 5 4 + 5 9 8 6 4 + 6 — 4 + 6 4 + 6 4 + 6 — 4 + 6 5 5

In the above table, ‘Max DL’ represents the maximum value of the number of DL subframes that are included in the section of k_(S-P)(n)+j_(P-S)(n), which is fixed by each subframe in the corresponding UL-DL configurations, and ‘Min DL’ represents the minimum value of the number of DL subframes that are included in the section of k_(S-P)(n)+j_(P-S)(n), which is fixed by each subframe in the corresponding UL-DL configurations.

In this case, the maximum number of DL HARQ processes that is applied to the secondary cell corresponds to one of the followings.

i) With the maximum value of the number of DL subframes available in sections corresponding to each k_(S-P)+j_(P-S), or the maximum value of the number of DL subframes included in sections corresponding to each k_(S-P)+j_(P-S), the maximum number of DL HARQ processes that is applied to the secondary cell may be determined. A valid DL subframe means a DL subframe that is available to transmit data by a base station. That is, if a base station is unable to transmit data, it is not a valid DL subframe even though it was set to be a DL subframe in the UL-DL configurations. In order for a specific DL subframe to be a valid DL subframe, data channel transmission should be available in the specific DL subframe and also, the control channel transmission should be available in the DL subframe that is defined to transmit the control channel that schedules the data channel.

ii) Or with the minimum value of the number of DL subframes available in sections corresponding to each k_(S-P)+j_(P-S), or the minimum value of the number of DL subframes included in sections corresponding to each k_(S-P)+j_(P-S), the maximum number of DL HARQ processes that is applied to the secondary cell may be determined.

For example, suppose a primary cell is the FDD cell and a secondary cell is the TDD cell based on UL-DL configuration 2. In this case, referring to Table 15, the value of k_(S-P)+j_(P-S) is 9 for each subframe of the secondary cells, and the maximum value of the number of all DL subframes included in sections corresponding to k_(S-P)+j_(P-S) respectively is 8, and the minimum value is 7. At the moment, the maximum number of DL HARQ processes of the secondary cell is set to be 8 or 7.

Such a method has a disadvantage that the DL HARQ process cannot be allocated in a part of DL subframes of the secondary cell, but there is an advantage in that data retransmission time can be shortened.

Or, the maximum number of DL HARQ processes of the secondary cell may be determined as follows. It is based on the premise that a primary cell (a first serving cell) is the FDD cell and a second cell (a second serving cell) is the TDD cell.

FIG. 22 illustrates an example of determining the number of DL HARQ processes of the secondary cell.

The UE receives data in a DL subframe of the secondary cell (the second serving cell), and transmits an ACK/NACK signal in response to the data in a UL subframe of the primary cell (the first serving cell). At the moment, a first type of frame (a FDD frame) is used in the primary cell, and a second type of frame (a TDD frame) is used in the secondary cell.

In this case, the number of DL HARQ processes of the secondary cell is calculated by the number of DL subframes that are included in each of sections including a specific number of subframes based on each subframe included in the TDD frame of the secondary cell (step, S171), and based on the number of the DL subframes, the number of DL HARQ processes of the secondary cell may be determined. Particularly, one of the following methods 1) to 4) can be applied.

1) In the FDD cell, the minimum time between the initial transmission and retransmission by the DL HARQ is eight subframes. Considering this, in the secondary cell, the maximum number of DL HARQ processes of the secondary cell may be determined by the maximum value of the number of DL subframes that are included in the section of eight subframes.

2) Or, the maximum number f DL HARQ processes of the secondary cell may be determined with the minimum value of the number of DL subframes that are included in the section of eight subframes in the secondary cell.

The following table represents the number of DL subframes that are included in the section of eight subframes for each subframe according to the UL-DL configurations when the secondary cell is the TDD cell.

TABLE 16 # of DL subframes within 8 subframes UL-DL Subframe n 0 1 2 3 4 5 6 7 8 9 Configuration 0 1 2 3 4 5 6 7 8 9 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Max Min 0 D D U U U D D U U U 4 3 2 3 4 4 3 2 3 4 4 2 1 D D U U D D D U U D 5 4 4 5 6 5 4 4 5 6 6 4 2 D D U D D D D U D D 6 6 6 7 7 6 6 6 7 7 7 6 3 D D U U U D D D D D 5 5 5 6 7 7 6 5 5 5 7 5 4 D D U U D D D D D D 6 6 6 7 8 7 6 6 6 6 8 6 5 D D U D D D D D D D 7 7 7 8 8 7 7 7 7 7 8 7 6 D D U U U D D U U D 4 3 3 4 5 5 4 3 4 5 5 3

Referring to the table 16, ‘# of DL subframes within 8 subframes’ represents the number of DL subframes included in the section of the corresponding 8 subframes depending on from which subframe the section of 8 subframes is started 0 to 9.

For example, if the section of 8 subframes is started from 0 based on UL-DL configuration 0 in Table 16, the number of DL subframes that are included in the section of 8 subframes is 4, and if the section of 8 subframes is started from 1, the number of DL subframes that are included in the section of the corresponding 8 subframes is 3. And, ‘Max’ represents the maximum value among the numbers of DL subframes that are included in each section of 8 subframes for the corresponding UL-DL configuration, and ‘Min’ represents the minimum value among the numbers of DL subframes that are included in each section of 8 subframes for the corresponding UL-DL configuration.

3) Or the maximum number of DL HARQ processes of the secondary cell may be determined based on an average value of the numbers of DL subframes that are included in each section of 8 subframes in the secondary cell. For example, in Table 16, UL-DL configuration 0 is applied for the secondary cell, the numbers of DL subframes that are included in each section of 8 subframes will be {4, 3, 2, 3, 4, 4, 3, 2, 3 and 4}. By obtaining an average value of these values, the nearest integer number of the average values may be determined to be the maximum number of DL HARQ processes of the secondary cell.

4) Or, a specific value between the maximum value and the minimum value of the numbers of DL subframes that are included in each section of 8 subframes in the secondary cell may be determined to be the maximum number of DL HARQ processes of the secondary cell. The specific value may be a predefined fixed value or a value signaled.

The more number of DL HARQ processes are there, the more utilization is available for the DL subframes that are included in the secondary cell. However, the soft buffer that stores data should be divided as many as possible. Therefore, there is a disadvantage in that it requires a big soft buffer. Considering such a tradeoff, a method may be chosen to be used from the methods 1) to 4) described above.

Or the maximum number of DL HARQ processes of the secondary cell may be determined to be the number of DL subframes that are included in the section of 10 subframes. Since an asynchronous HARQ is operated in DL, a base station freely set up the HARQ timing unlike a synchronous HARQ. For the sake of implementation, the maximum number of DL HARQ processes may be determined by the number of DL subframes that are included in the section of 10 subframes.

The following table represents the number of DL subframes that are included in the section of 10 subframes for the UL-DL configurations that are applied to the secondary cell.

TABLE 17 # of DL UL-DL subframes Config- Subframe n within 10 uration 0 1 2 3 4 5 6 7 8 9 subframes 0 D D U U U D D U U U 4 1 D D U U D D D U U D 6 2 D D U D D D D U D D 8 3 D D U U U D D D D D 7 4 D D U U D D D D D D 8 5 D D U D D D D D D D 9 6 D D U U U D D U U D 5

Or, the maximum number of the DL HARQ processes of the secondary cell may also be fixed to be eight, which is the same as that of the FDD cell. Particularly, this method may be applied in case that the FDD cell follows the ACK/NACK timing which is applied to the FDD cell when the FDD cell transmits the ACK/NACK for the data received in the DL subframe of the secondary cell, which is the TDD cell. This method has an advantage that the implementation is simple.

Or, the maximum number of the DL HARQ processes of the secondary cell may be determined to be the same value as the maximum number of the DL HARQ processes applied when the secondary cell, which is the TDD cell, unilaterally used. Or, throughout all UL-DL configurations in Table 16 above, the maximum or the minimum number of the DL subframes may be determined to be the number of the maximum DL HARQ processes of the secondary cell.

Or, when the maximum value that divides the soft buffer is M_(limit), the M_(limit) value may also be determined to be the maximum number of the DL HARQ processes of the secondary cell. If the number of the DL HARQ processes of the secondary cell is set to be greater than the maximum division number, it cannot be actually operated because it exceeds the capacity.

Or, the maximum number of the DL HARQ processes of the secondary cell, which is the TDD cell, may be limited to eight or less. In the DCI format that is scheduling the conventional FDD cell, 3 bits of HARQ process number field may be existed, and in the DCI format that is scheduling the TDD cell, 4 bits of HARQ process number field may be existed. If the maximum number of the DL HARQ processes of the secondary cell is limited to eight or less, 3 bits of HARQ process number field is included in the DCI format that is scheduling the TDD cell so as to be the same size of the DCI format that is scheduling the FDD cell.

In the methods described above, it has been described under the premise that a special subframe is applied as the DL subframe. However, this is not intended to impose a limitation, but the special subframe may be excluded when calculating the number of DL subframes included within a predefined subframe section. For example, the subframe in which DwPTS included in the special subframe is short as a predetermined value or less may be excluded.

In addition, in the methods described above, different methods may be combined and used for each UL-DL configuration. In addition, different methods may be applied according to whether the ACK/NACK timing is based on method 1 or method 2.

FIG. 23 is a block diagram of a wireless apparatus in which the embodiments of the present invention is implemented.

A base station 100 includes a processor 110, a memory 120 and a radio frequency (RF) unit 130. The processor 110 implements the proposed functions, processed, and/or methods. For example, the processor 110 setup a plurality of serving cells that use different frame structure with each other to a UE, and transmits data that requires ACK/NACK response through each of the serving cells. In addition, the processor 110 determines the number of the DL HARQ processes for the secondary cell. The memory 120 is connected to the processor 110 and configured to store various information used for the operations for the processor 110. The RF unit 130 is connected to the processor 110 and configured to transmit and/or receive a radio signal.

A UE 200 includes a processor 210, a memory 220, and a RF unit 230. The processor 210 implements the proposed functions, processed, and/or methods. For example, the processor 210 may receive the configuration of a first serving cell and a second serving cell that use different frame structures with each other, receive data in the DL subframe of the second serving cell, and transmit the ACK/NACK signal in response to the data in the UL subframe of the first serving cell. The number of DL HARQ processes of the secondary cell is determined based on a value which is signaled before or the number of DL subframe included in the subframe section of a predetermined number in the secondary cell.

The processor 110, 210 may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, data processing devices and/or converters for mutually converting baseband signals and radio signals. The memory 120, 220 may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The RF unit 130, 230 may include one or more antennas for transmitting and/or receiving radio signals. When an embodiment is implemented in software, the above-described scheme may be implemented as a module (process, function, etc.) for performing the above-described function. The module may be stored in the memory 120, 220 and executed by the processor 110, 210. The memory 120, 220 may be placed inside or outside the processor 110, 210 and connected to the processor 110, 210 using a variety of well-known means. 

What is claimed is:
 1. A method for performing hybrid automatic repeat request (HARQ) processes in a carrier aggregation system with which a plurality of serving cells are configured, the method performed by a user equipment (UE) and comprising: receiving a signal; determining a number of HARQ processes of a secondary cell among the plurality of serving cells based on the signal; and receiving physical downlink shared channels (PDSCHs) related to the HARQ processes on a secondary cell, wherein the signal informs the UE about the number of HARQ processes of the secondary cell.
 2. The method of claim 1, further comprising: transmitting acknowledgement/negative acknowledgement (ACK/NACK) information for data received in one of the PDSCHs.
 3. The method of claim 1, further comprising: wherein data is received through one of the PDSCHs in a downlink subframe of the secondary cell and acknowledgement/negative-acknowledgement (ACK/NACK) information for the data is transmitted in an uplink subframe of a primary cell.
 4. The method of claim 1, wherein the uplink subframe is determined by ACK/NACK timing of the primary cell.
 5. A user equipment (UE), comprising: a transceiver configured to transmit and receive a radio signal; and a processor connected to the transceiver, wherein the processor is configured to: receive a signal, determine a number of HARQ processes of a secondary cell among the plurality of serving cells based on the signal and receiving physical downlink shared channels (PDSCHs) related to the HARQ processes on a secondary cell, wherein the signal informs the UE about the number of HARQ processes of the secondary cell.
 6. The UE of claim 5, the processor is further configured to: transmit acknowledgement/negative acknowledgement (ACK/NACK) information for data received in one of the PDSCHs.
 7. The UE of claim 5, wherein data is received through one of the PDSCHs in a downlink subframe of the secondary cell and acknowledgement/negative-acknowledgement (ACK/NACK) information for the data is transmitted in an uplink subframe of a primary cell.
 8. The UE of claim 5, wherein the uplink subframe is determined by ACK/NACK timing of the primary cell. 